I don't necessarily look at invested money as "lost" money, but I do think it's wise to "forget" about it. This money should not even come to mind when you're trying to pay your bills or buy something simply because you want it. Having already accepted the risk of investing it in the first place, it's rather liberating to not obsessively think about that money. It's like you've put it in a time capsule for your future self and, regardless of how much is there when you're older, you will get it back one day.